Make engage_media_proxy branch-aware

Some time ago I had started the "Calling engage_media_proxy per branch"
thread. With latest changes to branch and reply routes I have been able
to achieve a setup handling Mediaproxy sessions mostly the way I liked
it to look like, however I was required to switch back to the old-style
way of manually calling use_media_proxy() and end_media_session():

* if caller has NAT issues, I add the dialog to a "mediaproxy"-profile
* in branch route, if nat_bflag is set, I set "use-mediaproxy"-bflag
* if either profile or branch is marked that way, I call mediaproxy
functions where required
* on 200 Ok and use-mediaproxy-bflag set (and "mediaproxy"-profile is
not), I add the dialog to the "mediaproxy"-profile

I'm still experimenting with some remaining little issues, but I'm
already really satisfied with the result.

As such a bullet-proof (needs still to be proven ;-)) branch-aware and
dialog-based setup is already possible to be achieved by me as a script
writer, I thought this would be the right time to re-launch my feature
request.

What I'd love to see: engage_media_proxy doing more or less what I've
mentioned above if called from branch routes.

Best regards,
Thomas Gelf